1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electrophotographic digital printer or copying machine.
2. Description of Related Art
Image forming apparatuses such as a laser printer use electrophotographic process are used practically, wherein a laser is driven according to digital image data converted from analog image signals and an image is reproduced with use of an electrophotographic process. Various digital image forming methods of reproducing a half-tone image such as a photograph with a high fidelity are proposed for such image forming apparatuses.
Digital image forming methods includes an area gradation method with use of a dither matrix, a multi-value laser exposure method such as a pulse width modulation method or an intensity modulation method, and a multi-value dither method which combines the dither method with a pulse width modulation method or an intensity modulation method. In the multi-value laser exposure method, the gradation is expressed by changing the pulse width (or light-emitting time) or the light intensity in order to change the light quantity of laser, that is, the light-emitting time times the intensity (refer for example Japanese Patent laid open Publications 62-91077/1987, 62-39972/1987, 62-188562/1987 and 61-22597/1987).
By using such a gradation method, the image density can be reproduced in principle in correspondence to the gradation of the image to be reproduced. However, the image density reproduced with a gradation method is not proportional correctly to the original document density due to the photosensitive characteristics of a photoconductor and the characteristics of toners. Then, the linear characteristic of the document density with the image density is not realized and the resultant nonlinear characteristic is called as the gamma characteristic generally. The nonlinearity is a large factor which lowers the fidelity of the reproduced image for a half-tone document.
Then, the so-called gamma correction is conducted to improve the gamma characteristic wherein the document density read with a sensor is converted with a conversion table for the gamma correction and a digital image is formed according to the converted document density to realize the linear characteristics. For example, in an apparatus with use of a digital image forming method such as an area gradation method or a multi-value laser exposure method disclosed in Japanese Patent laid open Publications 1-204741/1989 and 1-204743/1989, the gamma correction characteristic is changed according to a command from a host computer, an image reader or by a user. By using such a gamma correction, the image can be reproduced according to the document density level usually with a fidelity.
Further, there are other factors which affect the image density. The amount of adhered toners on the photoconductor on development changes according to the external environment such as temperature or humidity owing to the characteristics of the photoconductor and the toners. In general, in an environment of high temperature and high humidity, the adhered amount of toners increases, and the slope of the gamma characteristic at low and intermediate densities is known to increase so as to make the reproduced image darker. On the other hand, in an environment of low temperature and low humidity, the adhered amount of toners decreases, and the slope of the gamma characteristic at low and intermediate densities is known to decrease so as to make the reproduced image lighter. That is, the transfer efficiency varies with the environment conditions.
As mentioned above, the density of a reproduced image varies according to the environment. In order to solve this problem and to stabilize the reproduced image in a printer or a copying machine of electrophotographic process, the density is generally controlled so as to make the maximum density constant.
In an conventional automatic density control, a standard toner image as a standard for the density control is first formed on the surface of the photoconductor, and the density of the standard toner image is measured by detecting the quantity of reflection light with a sensor arranged near the photoconductor. The value detected by the sensor is received by a printer controller, which changes the grid potential of the sensitizing charger and the development bias potential of the developer according to the detected value with a prescribed value. Previously, the density control is performed by keeping the difference between the surface potential V.sub.o and the development bias potential V.sub.B constant in order to prevent a fog in the background of an image and to prevent the adhesion of carriers of two-component developer to the photoconductor.
However, though such density control can make the maximum density constant, it is a problem that the gamma characteristic changes largely when the surface potential V.sub.o and the development bias potential V.sub.B are changed by keeping the difference between the surface potential V.sub.o and the development bias potential V.sub.B constant. In other words, the reproduction image does not have the same gradation constantly for a document.
Further, it is also a problem that the initial quantity of light at which the image is reproduced first when the quantity of exposure light is increased from zero level varies with the process conditions. The process conditions include the surface potential V.sub.o due to the wear of the photoconductor drum (the accumulating total number of printing with use of the photoconductor) or to the scattering of the properties in the manufacturing process, the sensitivity of the photoconductor drum or of the light quantity of exposure, the transfer efficiency which changes according to the humidity or to the kind of a paper. Then, the number of the gradation levels which is reproducible changes, and the image quality changes largely at low densities which is sensitive to human eyes.
In order to solve such problems, previously, several measures are used. For example, the grid potential V.sub.G of the sensitizing charger is changed to control the surface potential V.sub.o to be a prescribed value. The temperature of the photoconductor and the quantity of exposure light are detected to control the quantity of exposure light. The temperature and the humidity are detected to control the transfer voltage.
However, it is expensive to use such measures automatically. If such measures are performed manually, the number of the parameters to be controlled is large so that it is not clear how to adjust the parameters surely and it is hard for a user to perform precise control.
As to the transfer efficiency, it is proposed that the transfer efficiency is controlled by the transfer current of the transfer charger which transfers the toner image of an electrostatic latent image on the photoconductor to a paper according to the absolute humidity or the temperature and the humidity (for example U.S. Pat. No. 4,912,515).
A factor which affects the transfer efficiency at high humidity is the decrease in electrical resistance due to the absorption of humidity by a paper. That is, the discharge current from the transfer charger leaks through the paper and the toner image is not transferred effectively. The leakage causes the injection of charges on the surface of the photoconductor. Thus, the injection of charges is liable to happen at high humidity. If charges are injected to the photoconductor, the surface of the photoconductor is charged at a high potential and the toners transferred to the paper are attracted to the charges on the photoconductor or the re-transfer to the photoconductor happens. The re-transfer affects largely the reproduced image. For example, a part of an image vanishes in the highlight portion of the reproduced image. In order to prevent the re-transfer, the transfer current of the transfer charger has to be decreased. However, this lowers the transfer efficiency. Therefore, it is difficult to stabilize the gradation to compensate the decrease in transfer efficiency according to the detected temperature and humidity because of the side effect of the re-transfer.
In order to compensate the decrease in transfer efficiency at high humidity, the light quantity may be changed. However, the change in the light quantity affect the entire gradation largely, while the change in the highlight portion is small. Especially, the initial light quantity at which the image is reproduced first when the quantity of exposure light is increased from zero level nearly varies with the light quantity of the laser light source. Therefore, it is difficult to compensate the decrease in transfer efficiency by changing the light quantity.
It is also a problem that the measured potential of the photoconductor is different from the potential which affects the deposition in the intensity modulation exposure method. Thus, a constant gradation characteristic is not reproducible for a document if the surface potential and the exposure light quantity are controlled according to the measured value of the potential.